Transducers are widely used in measurement and control systems for converting a physical quantity into a corresponding signal suitable for processing. A pressure transducer is a sensor which may respond to an applied fluid pressure and produce a signal (e.g., electrical, mechanical, or pneumatic) representative of the pressure. Typically, a pressure transducer may utilize a pressure-sensitive element where that element includes a portion having a position that varies with applied pressure. That position is transformed into an electrical signal representative thereof.
A particular class of pressure transducers employs a peripherally supported diaphragm as the pressure sensitive element, and operates in response to an applied pressure to translate a physical displacement of the diaphragm into an electrical signal. One prior art form of diaphragm transducers utilizes a capacitive-sensing arrangement for accomplishing the displacement-to-electrical signal conversion. FIG. 1 shows a prior art capacitive pressure sensor 10 including a relatively thin, edge-supported, electrically conductive diaphragm element 20 disposed across a concave base member 14 that houses a first distinct region 26 below diaphragm 20 and a second distinct region 33 above diaphragm 20. The two regions may be separately pressurized by couplings attached to pressure ports 12 and 16 to establish a pressure differential across diaphragm 20. The central portion of diaphragm 20 is movable in the direction of axis 14a in response to that pressure differential. Sensor 10 further includes an electrode element 32 within region 26 that serves as an electrically conductive element that is opposite and nominally separated by a distance d from a corresponding region of diaphragm 20. Accordingly, diaphragm 20 and electrode 32 effectively establish a "parallel" plate capacitor having a characteristic capacitance that varies inversely with d, which value is related to the pressure differential across diaphragm 20. Transducers of this configuration are described in U.S. Pat. No. 4,358,814 assigned to Setra Systems, Inc., the assignee of the present invention. Sensors of this type thus produce an electrical characteristic (i.e., capacitance between electrode 32 and the diaphragm) that is representative of the distance d, which in turn is representative of the pressure differential across the diaphragm; that differential may be established by either gas or liquids in two respective regions on either side of the diaphragm.
Diaphragm-based pressure transducers are useful in industrial applications requiring pressure measurements of a liquid column enclosed within a pressurized cell. For example, the height of the liquid column may be derived from the measured column pressure. However, conventional level-measuring apparatus are currently limited in their ability to precisely measure fluid levels in such cells, creating particular difficulties for biomedical applications where significant bioactive processing occurs in these cells. The conventional apparatus described below are noteworthy for such problems as nonsanitary couplings to the interior of the cell and requirements for costly high dynamic range sensors.
In a typical closed, pressurized cell or tank system where it is desired to measure the level of a liquid in the tank, there are two component pressures of interest: one corresponding to the "total pressure," that is, the pressure at the bottom of the tank due to the weight of the liquid as burdened by the blanket pressure (i.e., the pressure in the region above the liquid), and the other corresponding to the blanket pressure. One conventional level measurement system is shown in FIG. 2A for a pressurized cell or tank 19 enclosing a liquid column 21 and an overlying blanket region 20. The illustrated system employs a combination of pressure sensors (exemplified by the illustrated diaphragm pressure transducers and associated sensor networks) that independently measure total pressure and blanket pressure. The difference between the independent measurements is reported as the pressure of the liquid column. As shown, an upper pressure transducer 22, positioned at or near the top of tank 19, includes a diaphragm 23 flush to the cell wall for detecting the pressure of blanket region 20. The resultant pressure is converted to an electrical signal by associated sensor network 24, producing an output on line 25. A similar pressure transducer 26, positioned at or near the bottom of tank 19, includes a diaphragm 27 flush to the cell wall for detecting the total pressure of the cell, namely the pressure of liquid column 21 plus the pressure of blanket region 20. This pressure differential is convened to an electrical signal by associated sensor network 28, producing an output on line 29. The signals on lines 25 and 29 may be differenced to produce a signal representative of the level of liquid column 21.
The gauge-type pressure measurements of FIG. 2A require that transducer 22 and associated network 24 have a full scale range at least as high as the maximum blanket pressure, and that the transducer 26 and associated network 28 have a full scale range at least as high as the total cell pressure. This requirement therefore demands that both transducers have a large dynamic range since the pressure in the blanket region may vary widely as the liquid level moves from the top of the tank to the bottom. For example, the liquid column may produce a pressure of only 4.+-.0.5 psi over the range of levels, while the blanket pressure may vary in the range of 20-50 psi, an appreciable contribution to the total cell pressure. Since the overall accuracy of the sensor's measurement capability is a function of the accuracy of the individual pressure sensors, which in turn is specified as a percentage of full range, a high accuracy system may only be constructed using costly, large dynamic range sensors.
FIG. 2B illustrates another conventional level sensing system similarly having two flush diaphragm sensing elements, but employing liquid-filled capillary tubing to couple the diaphragm displacements to a single differential diaphragm transducer 31 and an associated sensor network 32. The system comprises an upper isolation assembly 22 including a diaphragm 23 flush to an upper portion of the cell wall for detecting the pressure of blanket region 20, and further comprises a lower isolation assembly transducer 26 including a diaphragm 27 flush to a lower portion of the cell wall for detecting the total pressure of the cell. In this configuration, the pressure detected by each of diaphragms 23 and 27 is hydraulically transmitted to the diaphragm 31A of transducer 31 using a respective one of silicone oil-filled capillary tubes 30A and 30B. This connectivity to the remotely-positioned transducer 31 introduces the risk of media contamination due to leakage of silicone oil from transducer 31 or the connecting capillary tubes or isolation assemblies 22 and 26. Additionally, any physical displacement or thermally-induced unbalanced expansion of the liquid-filled tubes will cause erroneous pressure signals to be presented to the diaphragm 31A, thereby leading to faulty or inaccurate readings at the output 32A.
FIG. 2C shows an additional prior art level measuring system utilizing a single differential diaphragm transducer 31 that senses the blanket pressure, as coupled by pneumatic line 34, with respect to the fluid pressure created by the displacement of a flush diaphragm 27, as coupled by silicone oil-filled capillary tube 30B, for detecting the total pressure of the cell. In this configuration, the pressure of blanket region 20 is communicated to sensor 31 via line 34 through a non-flush, non-sanitary reference pressure port of sensor 31. The capillary attachment is a drawback to this configuration because it exposes the media within the cell to additional environments (e.g., tubing) external to the cell. This configuration is thus precluded from any application requiring absolute clean-in-place (CIP) or steam-in-place (SIP) sanitation processing.
FIG. 2D is a yet further prior art system that is similar to the FIG. 2C apparatus, but replaces the nonflush/nonsanitary reference port and capillary tube 34 with a pneumatic pressure repeater 36 coupled to pressure transducer 22 having flush diaphragm 23. The pressure repeater 36 provides a secondary pneumatic signal to the blanket input port of differential sensor 31. This configuration avoids problems created by the configuration of FIG. 2C due to the non-flush, non-sanitary port. However, although the pneumatic pressure repeater 36 is a relatively reliable instrument, there is no mechanism in this configuration for eliminating or compensating for any errors arising from the signal to pneumatic conversion; rather, these conversion errors combine additively with the total system inaccuracy. Any such error elimination or compensation would require a sophisticated, and correspondingly costly, pressure repeater having a sufficient degree of accuracy to avoid or overcome the additive effect of any conversion errors, thus adding an undesirable level of complexity to this configuration.
Accordingly, it is an object of the present invention to provide an improved liquid level measuring system for a pressurized cell.
Another object is to reduce the dynamic range required of the pressure sensors in such systems and to eliminate the use of capillary tubing in constructing a fully sanitary measuring system.
It is a further object of the present invention to reduce measurement errors that may be introduced by any of the elements or instrumentalities used in a liquid level measuring apparatus.